Masks for microlithography and methods of making and using such masks

ABSTRACT

Masks for microlithography apparatus, methods for making such masks, and methods for exposing photosensitive materials to form arrays of microfeatures on semiconductor wafers using such masks. In one embodiment, a method of making a mask comprises forming a mask layer on a substrate and identifying a first opening in the mask layer corresponding to a first feature site at which an intensity of the radiation at a focal zone is less than the intensity of the radiation at the focal zone for a second feature site corresponding to a second opening in the mask. The second opening is adjacent or at least proximate the first opening. The method can further include forming a first surface at the first opening and a second surface at the second opening such that radiation passing through the second opening constructively interferes with radiation passing through the first opening at the focal zone.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/167,316 filed Jun. 23, 2011, which is a continuation of U.S.application Ser. No. 12/917,249 filed Nov. 1, 2010, now U.S. Pat. No.7,972,753, which is a continuation of U.S. application Ser. No.11/838,130 filed Aug. 13, 2007, now U.S. Pat. No. 7,838,178, each ofwhich is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure is related to masks for use in microlithographytools, and methods of making and using such masks.

BACKGROUND

Microelectronic features are typically formed in and/or on semiconductorwafers or other microfeature workpieces by selectively removing materialfrom the wafer and depositing insulative, semiconductive and/orconductive materials onto the wafer. Microlithography is widely used insemiconductor processing to pattern small features onto the wafer. Atypical microlithography process includes depositing a layer ofradiation-sensitive material on the wafer (e.g., photoresist),positioning a patterned mask or mask over the photoresist layer, andthen irradiating selected regions of the masked photoresist layer with aselected radiation. The wafer is then exposed to a developer, such as anaqueous base or a solvent, that removes either the irradiated regions orthe non-irradiated regions of the photoresist layer. In one case, thephotoresist layer is initially generally soluble in the developer, andthe portions of the photoresist layer exposed to the radiation throughpatterned openings in the mask change from being generally soluble tobeing generally resistant to the developer (e.g., so as to have lowsolubility). Alternatively, the photoresist layer can be initiallygenerally insoluble in the developer, and the portions of thephotoresist layer exposed to the radiation through the openings in themask become more soluble. In either case, the portions of thephotoresist layer that are resistant to the developer remain on thewafer, and the rest of the photoresist layer is removed by the developerto form a pattern of openings in the photoresist layer.

After forming the openings in the photoresist layer, the wafer oftenundergoes several etching or deposition processes. In an etchingprocess, the etchant removes material by the openings in the photoresistlayer, but not material protected beneath the remaining portions of thephotoresist layer. Accordingly, the etchant creates a pattern offeatures (such as grooves, channels, or holes) in the wafer material orin materials deposited on the wafer. These features can be filled withinsulative, conductive, or semiconductive materials in subsequentdeposition processes to build layers of microelectronic features on thewafer. In other deposition processes, metals or other materials can beplated into the openings of the photoresist layer using electroless orelectrolytic techniques. The wafer is subsequently singulated after suchprocessing to form individual chips, which can be incorporated into awide variety of electronic products, such as computers and otherconsumer or industrial electronic devices.

Microlithography can be a limiting factor in circuit design because itis challenging to pattern progressively smaller circuit features whilestill maintaining cost-effective manufacturing. For example, certainsites for contacts in high density contact arrays may have a reduceddepth of focus such that they do not have the same contrast level asadjacent contacts. This limits the spacing between the contacts suchthat there is a minimum pitch for dense contact patterns, which in turnlimits the ability to decrease the die sizes and/or the cost ofmanufacturing semiconductor products.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a portion of a mask foruse in a microlithography apparatus in accordance with an embodiment ofthe invention.

FIG. 2 is a schematic cross-sectional view of a portion of a mask foruse in a microlithography apparatus in accordance with anotherembodiment of the invention.

FIG. 3 is a schematic cross-sectional view of a portion of a mask foruse in a microlithography apparatus in accordance with yet anotherembodiment of the invention.

FIG. 4 is a flow chart illustrating a method of making a mask inaccordance with an embodiment of the invention.

FIG. 5 is a flow chart of a method for exposing a photosensitivematerial to form an array of microfeatures on a semiconductor wafer inaccordance with an embodiment of the invention.

FIG. 6 is a flow chart of a method for exposing a photosensitivematerial to form an array of microfeatures on a semiconductor wafer inaccordance with an embodiment of the invention.

DETAILED DESCRIPTION

The following disclosure describes several specific embodiments of masksfor microlithography apparatus, methods for making such masks, andmethods for exposing photosensitive materials to form arrays ofmicrofeatures on semiconductor wafers using such masks. Although themicrofeatures are described below with reference to contacts, themicrofeatures can be trenches, doped regions, or other features that aremanufactured on or in semiconductor wafers. Additionally, even thoughthe processes are described with reference to processing semiconductorwafers, the processes can be used on other substrates upon which and/orin which microelectronic devices, micromechanical devices, data storageelements, optics, read/write components, and other features arefabricated. For example, SRAM, DRAM (e.g., DDR/SDRAM), flash memory(e.g., NAND flash memory), processors, imagers, and other types ofdevices can be constructed on semiconductor wafers or other substrates.Moreover, several other embodiments of the invention can have differentconfigurations, components, or procedures than those described in thissection. A person of ordinary skill in the art, therefore, willaccordingly understand that the invention may have other embodimentswith additional elements, or the invention may have other embodimentswithout several other features shown and described below with referenceto FIGS. 1-6.

FIG. 1 is a schematic cross-sectional view of a portion of a mask 10 inaccordance with an embodiment of the invention. The mask 10 is generallya component of a reticle used in scanners, steppers, or other types ofmicrolithography tools that pattern very small features onto wafers. Itwill be appreciated that the features of the mask are not drawn to scalein FIGS. 1-3.

In the embodiment shown in FIG. 1, the mask 10 includes a substrate 20having a front side 22 configured to face toward a wafer and a back side24 configured to face a radiation source or transmitter. The substrate20 can be quartz or another material transmissive to a radiation Rdirected toward the back side 24 of the substrate 20. The mask 10further includes a mask layer 30 at the front side 22 of the substrate20. The mask layer 30 can be chrome or another material that is opaque,non-transmissive or semi-transmissive to the radiation R. The mask layer30 has a plurality of openings including at least one first opening 32and at least one second opening 34. The openings 32 and 34 are arrangedin a desired configuration of microfeatures that are to be patterned ona workpiece W. The mask layer 30, for example, can have a plurality offirst and second openings 32 and 34 arranged to form an array ofcontacts at contact sites C at the workpiece W. As such, the openings 32and 34 are configured to have a dimension D and a pitch P betweenopenings such that radiation passing through the openings 32 and 34irradiates exposed regions E in a photoimageable layer L correspondingto the contact sites C.

The radiation intensity at contact sites may vary at locations across anarray using conventional masks such that the radiation intensity atcertain contact sites is not sufficient. The contrast at the borders ofthe exposed regions of the photoresist layer corresponding to suchlow-radiation contact sites is less than at other exposed regions, andthis can adversely affect the uniformity of the size and shape of theresulting contacts. The present inventors believe that the first orderdiffractions from certain openings in the mask layer 30 destructivelyinterfere with other openings such that only selected openings have alower radiation intensity.

To resolve this problem, the mask 10 is configured such that radiationpassing through the second openings 34 constructively interferes withradiation passing through an adjacent first opening 32 that wouldotherwise be subject to destructive interference from the radiationpassing though the second openings 34. As a result, the radiationintensity at contact sites C associated with first openings 32 isaugmented by the radiation associated with one or more adjacent secondopenings 34 instead of being diminished by the radiation from theneighboring second openings 34. More specifically, in the particularembodiment illustrated in FIG. 1, the mask 10 has first surfaces 41 atcorresponding first openings 32 and second surfaces 42 at correspondingsecond openings 34 that are configured so that radiation exiting thesecond surfaces 42 constructively interferes with radiation exitingadjacent first surfaces 41 at the workpiece W. In the embodiment shownin FIG. 1, the first surfaces 41 are recessed into the substrate 20 at alocation between the front side 22 and the back side 24, and the secondsurfaces 42 are proximate to the front side 22. The first surfaces 41are accordingly offset relative to the second surfaces 42 by an offsetO, which is selected to create a phase shift between the radiationpassing through the first openings 32 and the radiation passing throughthe second openings 34. The offset O, for example, can be selected suchthat first order diffractions of the radiation passing through thesecond openings 34 constructively interfere with the radiation passingthrough the adjacent first openings 32. In a specific embodiment, theoffset O is such that the phase of the radiation exiting the firstsurfaces 41 is shifted 180° from the phase of the radiation exiting thesecond surfaces 42. When the phase of the radiation passing through thefirst openings 32 is shifted 180° from the phase of the radiationpassing through the second openings 34, the offset O can be determinedaccording to the following equation:

$O = \frac{\lambda}{2\left( {n - 1} \right)}$where λ is the optical wavelength for the radiation and n is therefractive index of the substrate 20.

In operation, the mask 10 is positioned relative to the workpiece W sothat the first and second openings 32 and 34 are aligned with thecorresponding contact sites C. The radiation R passes through the firstand second openings 32 and 34 to accurately irradiate the exposedregions E in the photoimageable layer L. For example, the radiationpassing through the first openings 32 is augmented by the radiationpassing through the second openings 34 within a focal zone at a focalrange F_(d) from the mask 10. As such, when the first openings 32 areselected to be at contact sites that would otherwise have a lowerradiation intensity, the mask 10 enhances the radiation intensity atsuch contact sites associated with corresponding first openings 32. Thisin turn enhances the contrast of the exposed regions E associated withcorresponding first openings 32. As a result, the first and secondopenings 32 and 34 can be spaced closer to each other compared toconventional masks to enable accurate patterning of high density arraysof microfeatures.

FIG. 2 illustrates another embodiment of the mask 10 in which the firstopenings 32 still correspond to contact sites subject to a lowerradiation intensity relative to the contact sites associated with thesecond openings 34. In this embodiment, the second surfaces 42 arerecessed into the substrate 20 at a location between the front side 22and the back side 24, and the first surfaces 41 are at least proximateto the front side 22. The second surfaces 42 can be recessed into thesubstrate 20 by the same offset O as explained above with respect toFIG. 1. The embodiment of the mask 10 illustrated in FIG. 2 accordinglyoperates such that the first order diffractions of radiation passingthrough the second openings 34 constructively interfere with theradiation passing through the first openings 32 to increase theradiation intensity at the contact sites C aligned or otherwiseassociated with the first openings 32.

The embodiments of the mask 10 shown in FIGS. 1 and 2 can be constructedby forming the mask layer 30 on the substrate 20 such that the masklayer has at least one first opening 32 and a plurality of secondopenings 34 arranged in an array corresponding to a desired pattern offeatures to be formed in a die of the workpiece W. The first opening 32is selected to be at a contact site where the radiation intensity in thefocal zone would otherwise be less than that at adjacent contact siteswithout having the appropriate offset between the first and secondsurfaces 41 and 42. Several embodiments of the method accordinglyinclude identifying the location(s) of the feature site(s) where theradiation intensity is less than that at adjacent sites or other siteswithout an offset between surfaces in the openings. The first opening(s)can be identified by, for example, etching all of the openings in themask layer 30, passing radiation through the openings before formingrecesses at the front side 22 of the substrate 20, and measuringradiation intensities at feature sites in the focal zone. Each firstopening can be selected to correspond to a site having a lower measuredradiation intensity than adjacent sites. In another embodiment, thefirst opening can be identified by modeling radiation intensities atsites in the focal zone and selecting a site having a lower radiationintensity than adjacent feature sites. After identifying the firstopenings 32 among the openings in the mask layer 30, the first andsecond surfaces 41 and 42 are offset relative to each other such thatradiation passing through the second openings 34 constructivelyinterferes with radiation passing through an adjacent first opening 32.In the embodiment of the mask 10 shown in FIG. 1, for example, the firstsurface 41 is formed by etching the substrate 20 through the firstopenings 32 as known in the art. The embodiment illustrated in FIG. 2can be formed by etching the second surfaces 42 into the substrate 20.

FIG. 3 illustrates a mask 100 in accordance with another embodiment ofthe invention. The mask 100 shown in FIG. 3 has several features incommon with the mask 10, and thus like references numbers refer to likecomponents in FIGS. 1-3. The mask 100, however, has a phase shiftelement 140 in selected openings to selectively shift the phase of theradiation passing through the openings relative to other openings. Inthe specific embodiment shown in FIG. 3, the mask 100 has phase shiftelements 140 in the first openings 32. The phase shift elements 140define first surfaces 141 in corresponding first openings 32, and thefront side 22 of the substrate 20 defines second surfaces 142 incorresponding second openings 34. The phase shift elements 140 arecomposed of a material that shifts the phase of the radiation passingthrough the first openings 32 relative to the radiation passing throughthe second openings 34 based on the different properties of thematerials used for the substrate 20 and the phase shift elements 140. Inan alternative embodiment, the phase shift elements 140 can define thesecond surfaces 142 in the second openings 34 to shift the phase of theradiation passing through the second openings 34 relative to theradiation passing through the first openings 32. In still anotherembodiment, first phase shift elements can be in the first openings 32and second phase shift elements with properties different than the firstphase shift elements can be in the second openings.

FIG. 4 is a flow chart illustrating a method 400 of making a mask foruse in microlithography tools to fabricate semiconductor devices inaccordance with another embodiment. The method 400 includes forming anarray of openings through a mask layer on a substrate (block 410). Thearray of openings can correspond to a pattern of features of a die on asemiconductor wafer, and the openings include at least one first openingand a plurality of second openings. The method 400 further includesforming a surface in one of the first or second openings (block 420).The surface is configured such that radiation passing through the firstopening is out of phase relative to the radiation passing through thesecond opening.

FIG. 5 is a flow chart of a method 500 of exposing photosensitivematerial to form an array of microfeatures on a semiconductor wafer. Themethod 500 includes identifying a first site for a first feature and asecond site for a second feature in an array of features of a die on thewafer (block 510). The first site is subject to receiving less radiationor otherwise having a lower radiation intensity than the second site.The method 500 further includes positioning a wafer at a focal zone forradiation passing through a mask (block 520) and irradiating the firstand second sites (block 530). The first and second sites are irradiatedsuch that radiation corresponding to the second site constructivelyinterferes with radiation corresponding to the first site at the focalzone.

FIG. 6 is a flow chart illustrating a method 600 for exposingphotosensitive material to form contacts on a semiconductor wafer. Themethod 600 includes positioning first and second openings of a mask toirradiate first and second sites, respectively, of an array of featuresof a die on a semiconductor wafer (block 610). The method 600 furtherincludes passing radiation through the first and second openings (block620). The radiation is passed through the first and second openings suchthat first order diffractions of radiation passing through the secondopenings constructively interfere with radiation passing through thefirst opening at a desired focal zone (block 630).

From the foregoing, it will be appreciated that specific embodiments ofthe invention have been described herein for purposes of illustration,but well-known structures and function have not been shown or describedin detail to avoid unnecessarily obscuring the description of theembodiments of the invention. Where the context permits, singular orplural terms may also include the plural or singular term, respectively.Moreover, unless the word “or” is expressly limited to mean only asingle item exclusive from the other items in reference to a list of twomore items, then the use of “or” in such a list is to be interpreted asincluding (a) any single item in the list, (b) all of the items in thelist, or (c) any combination of the items in the list. Additionally, theterm “comprising” is used throughout to mean including at least therecited feature(s) such that any greater number of the same featureand/or additional types of features are not precluded.

It will further be appreciated that various modifications to theforegoing embodiments may be made without deviating from the inventions.For example, many of the elements of one embodiment can be combined withother elements in addition to, or in lieu of, the elements in the otherembodiments. For example, the phase shift elements 140 illustrated inFIG. 3 can be combined with any of the openings of the embodimentsillustrated in FIGS. 1 and 2. Accordingly, the invention is not limitedexcept as by the appended claims.

We claim:
 1. A microlithography mask for illuminating a photoimageablematerial on a semiconductor wafer, comprising: a radiation transmissivesubstrate having a front side and a back side, the substrate having aplurality of recesses extending from the front side to a correspondingdepth into the substrate along walls of the recesses, wherein therecesses are arranged in an array; and a mask material at the front sideof the substrate, the mask material having openings arranged inrows/columns of the array, the openings corresponding to features to beformed on a wafer, the openings being at least partially aligned withthe recesses and non-recessed portions on the substrate, wherein theopenings are arranged at a constant pitch in the rows or in the columns,wherein a distance from one opening to another and the depth of therecesses are selected at least in part based on determining aninterference of radiation at a focal distance from the openings, andwherein shortest distance between the walls of adjacent recesses variesfor the recesses in the array, and wherein at least two non-recessedportions in a row or in a column are adjacent.
 2. The mask of claim 1wherein the interference of light is configured to occur on thephotoimageable material facing the mask material at the focal distancerange.
 3. The mask of claim 2 wherein the interference of lightprojected from the openings produces at least one constructiveinterference at the focal distance range.
 4. The mask of claim 2 whereinthe interference of light projected from the openings produces at leastone destructive interference at the focal distance range.
 5. The mask ofclaim 1 wherein the openings comprise a plurality of first openingsgenerally aligned with the corresponding recesses in the substrate and aplurality of second openings.
 6. The mask of claim 5 wherein: the depthof the recesses is determined such that the radiation leaving thecorresponding first openings is phase-shifted approximately 180° fromthe radiation leaving the second openings.
 7. The mask of claim 6wherein a distance that causes the radiation exiting the first surfaceto be approximately 180° out of phase relative to the radiation exitingthe second surface is determined as${O = \frac{\lambda}{2\left( {n - 1} \right)}},$ where λ is an opticalwavelength for the radiation, and n is a refractive index of thesubstrate.
 8. The mask of claim 5 wherein the depth of the recesses anda distance between a pair of adjacent first and second openings isdetermined at least in part based on observed difference between theintensity of radiation on a corresponding pair of feature sites on thephotoimageable material.
 9. The mask of claim 5, further comprising aphase shift element in one of the first and second openings.
 10. Themask of claim 5, further comprising a first phase shift element at leastone first opening.
 11. The mask of claim 10, further comprising a secondphase shift element at at least one second opening, wherein the firstphase shift element and the second phase shift element have differentphase shifting properties.
 12. A microlithography mask for illuminatinga photoimageable material on a semiconductor wafer, comprising: a maskmaterial having first and second openings in rows and columns of anarray corresponding to features to be formed on a wafer, wherein theopenings are arranged at a constant pitch in the rows or in the columns;and a radiation transmissive substrate having a front side facing themask material and a back side opposite from the front side, thesubstrate having a plurality of recesses aligned with the first openingsin the mask material, and a plurality of non-recessed areas at the frontside, the non-recessed areas aligned with the second openings in themask material, the recesses extending from the front side to acorresponding depth defined by a first surface in the substrate; andwherein a distance from one opening to another and the depth of therecesses are determined based on— a first radiation intensity at a firstlocation of the photoimageable material in a focal zone corresponding toa first opening, and a second radiation intensity at a second locationon the photoimageable material in the focal zone corresponding to asecond opening, wherein constructive or destructive interference occursat at least one of the first and second locations on the photoimageablematerial, and wherein at least one first opening has: another firstopening adjacent to a first side of the at least one first opening, asecond opening adjacent to a second side of the at least one firstopening, the second side of the at least one first opening beingopposite from the first side of the at least one first opening, andwherein at least two second openings in a row or in a column areadjacent.
 13. The mask of claim 12, further comprising a first phaseshift element at at least one first opening.
 14. The mask of claim 12,further comprising a second phase shift element at at least one secondopening, wherein the first phase shift element and the second phaseshift element have different phase shifting properties.